A metered-dose inhaler (MDI) is a device that delivers a measured quantity of aerosolized medication.
Referring now to FIG. 1, one illustrative example of a metered-dose inhaler, indicated generally by reference 100, will be described. The metered-dose inhaler 100 of FIG. 1 is made up of three primary components: a canister 102, a metering valve 104 and an actuator 106. The canister 102 is typically formed from stainless steel or aluminum, and contains the material to be dispensed; i.e. the medication 108 and propellant 110 (other materials such as excipients may also be included). The metering valve 104 is sealed to the canister 102, and includes a movable hollow valve stem 112. The metering valve 104 is configured so that when the valve stem 112 is moved toward the canister 102 from a containing position into a dispensing position (i.e. when the valve is actuated), a metered quantity of the medication 108 is released from the canister 102 through the valve stem 112. The configuration of the metering valve 104 is such that even if the valve stem 112 is maintained in the dispensing position, only the metered quantity of medication 108 is dispensed. Design and construction of metering valves is well known, and is not discussed further. The actuator 106 comprises a hollow body 114 that receives the canister 102, a mouthpiece 116, typically projecting obliquely from the body 114, and an actuator nozzle 118, also referred to as an atomizing nozzle, projecting inwardly at the junction of the body 114 and the mouthpiece 116. The valve stem 112 is received by the actuator nozzle 118 in fluid communication therewith so that pushing the canister 102 toward the actuator nozzle 118 moves the valve stem 112 (relative to the canister 102) into the dispensing position and releases the metered quantity of medication 108 into the actuator nozzle 118. The actuator nozzle 118 is configured to generate a plume 120 from the contents of the metered-dose inhaler canister 102 received through the valve stem 112 and direct the plume 120 through the mouthpiece 116. A patient would administer a dose of the medication 108 by pressing the canister 102 into the body 114 of the actuator 104 while inhaling through the mouthpiece 106.
Inhaling directly from a metered-dose inhaler can be difficult, and patients may use a tube having a mouthpiece at one end and a receptacle that receives the actuator mouthpiece 116 at the other end. These tubes, referred to as holding chambers or spacers, function as a reservoir to contain the metered dose inhaler plume 120, making it easier to inhale. However, such holding chambers or spacers are generally large and cumbersome.